Hi,
I hope I am understanding the questions correctly:
1 - When you click One-Time update a new window will open to configure the settings for the one time update that it will start as soon as you configure those settings. So it not automatically starts the installation, it starts the process of configuring that and you still have one last step to confirm that you want to start it.
2- It is still automatic patch installation meaning you do not login to the machines to install them. Just the process to start the installation is not scheduled but triggered manually now.
3 - You can one-time update one machines or many machines. You can schedule updates for one machine or many machines. Whatever you choose it is up to your personal preference. Usually scheduled updates are used to have monthly process of apply the updates so you know in advance on which date the patches will be applied. One time update is usually used when you have some urgent update that you want to apply now or you have just several machines that you manage on your own.
4- How long it takes depends on the number of patches that are installed and which patches are installed. Also if you have restarts or not. You cannot predict that in advance. You can set maximum maintenance window let's say 120 minutes. Once the installation starts it will stop applying patches around 120 minutes after that to avoid breaking that maintenance window. The maintenance window can be set on one-time updates and scheduled updates as well.
5- You can use one-time updates and scheduled deployments but be careful not to start those two at the same time. Most likely the one that is second will fail but still does not makes sense to do one-time updates at the same time the scheduled updates are running.
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